A set of 10 cards consists of five red cards and five black cards. The cards are shuffled thoroughly, and I choose one at random, observe its color, and replace it in the set. The cards are thoroughly reshuffled, and I again choose a card at random, observe its color, and replace it in the set. This is done a total of four times. Let X be the number of red cards observed in these four trials. The mean of X is: